1. Field of the Invention
The present invention relates to a sheet processing apparatus configured to implement a process on a sheet, and to an image forming apparatus.
2. Description of the Related Art
Hitherto, some image forming apparatuses, e.g., a copier, a laser printer, a facsimile, and a multi-function copier include a sheet processing apparatus configured to implement various processes such as binding (stapling) and sorting sheets on which image have been formed. One sheet processing apparatus widely used among such sheet processing apparatuses is provided with an intermediate processing tray therein, forms a bundle of sheets (referred to as a ‘bundle sheet’ hereinafter) by stacking a plurality of sheets on the intermediate processing tray, and implements a binding process on the sheet bundle.
Such sheet processing apparatus requires a certain processing time in implementing the binding process on the sheets. Then, there is a case when the processing time exceeds a sheet discharge interval without completing the binding process within a period from when a final sheet of the sheet bundle to be processed is discharged to the intermediate processing tray until when a next sheet is discharged to the intermediate processing tray, though it also depends on image forming speed of the image forming apparatus that outputs the sheets to the sheet processing apparatus. In such a case, although it is necessary to interrupt an image forming process of a succeeding sheet to complete the binding process of the preceding sheet bundle, productivity of the image forming apparatus drops if the image forming process is interrupted.
Then, Japanese Patent Application Laid-open No. 2010-173758 has disclosed a sheet processing apparatus configured to implement a buffering process of temporarily making several leading sheets of a succeeding sheet bundle stand by during when a binding process is implemented on a preceding sheet bundle on the intermediate processing tray for example. Specifically, this sheet processing apparatus is provided with a branch path branched from a conveying path for conveying a sheet and is configured to make a sheet stand by at the branch path in implementing the buffering process. When the sheet processing apparatus makes a plurality of sheets stand by, the sheet processing apparatus returns the sheet(s) standing by in the branch path from the branch path to the conveying path concurrently with a succeeding sheet to be conveyed and makes those overlapped sheets stand by at the branch path.
By the way, the image forming speed of the image forming apparatus is increasing year by year. Therefore, it is required to increase a number of sheets to be overlapped in the buffering process to assure a time for the binding process in the buffering process. However, if the number of sheets to be overlapped increases, a number of times when the sheets move in and out of the branch path described above also increases. Here, because the sheet receives conveyance resistance from the branch path in moving in and out of the branch path, a shift length between the overlapped sheets increases from a predetermined shift length corresponding to an increase of the number of times of the move of the sheets that move in and out of the branch path. The conveyance resistance is considered to be caused by sliding friction of the sheet that slides along a conveying guide for example, and the shift length between the overlapped sheets increases by the sliding friction of the sheets that slide along a stationary guide of the branch path. Accordingly, the more the number of times of the move of the sheets that move in and out of the branch path, the more the shift length between the sheets increases proportionally with a number of times of receiving the sliding friction caused with the stationary guide. That is, a sheet overlapped preceding to a finally overlapped sheet among the overlapped sheets including the finally overlapped sheet and the previously overlapped sheet, receives an influence of the sliding friction caused with the stationary guide by one time more than that received by the final sheet, and the more the sheet is previously overlapped, the more the shift length from the sheet overlapped thereafter increases.
Due to that, if the number of sheets to be overlapped increases, the sheet processing apparatus described above has a possibility of increasing the shift length between the sheets, of increasing a sheet overlapping length more than a predetermined sheet overlapping length preset to be able to align the sheets in discharging to the processing tray, and of thus causing misalignment.